In a liquid crystal display element, a classification based on an operating mode for liquid crystals includes phase change (PC), twisted nematic (TN), electrically controlled birefringence (ECB), in-plane switching (IPS), vertical alignment (VA) and so forth. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transreflective type utilizing both the natural light and the backlight.
A liquid crystal composition having a nematic phase and having suitable characteristics is used for the devices. General characteristics of the liquid crystal composition should be improved to obtain an AM device having good characteristics. A relationship in the general characteristics of the AM device and the liquid crystal composition is shown in Table 1 below.
The general characteristics of the liquid crystal composition will be explained further based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is 70° C. or higher and a preferred minimum temperature of the nematic phase is −10° C. or lower. Viscosity and a rotational viscosity of the liquid crystal composition relate to a response time in the device. Therefore, a short response time is preferred for displaying moving images on the device. Accordingly, the viscosity of the liquid crystal composition is preferably as small as possible. Furthermore, the viscosity is further preferably as small as possible also at a low temperature.
TABLE 1General Characteristics of Liquid Crystal Composition and AM DeviceGeneral Characteristics ofGeneral CharacteristicsNo.Liquid Crystal Compositionof AM Device1wide temperature range ofwide usable temperature rangea nematic phase2small viscosity1)short response time3suitable optical anisotropylarge contrast ratio4large positive or negativelow threshold voltagedielectric anisotropyand small electricpower consumptionlarge contrast ratio5large specific resistancelarge voltage holding ratio andlarge contrast ratio6high stability to ultraviolet lightlong service lifeand heat7small content of impurityunlikely poor display1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
An optical anisotropy of the liquid crystal composition relates to a contrast ratio in the device. Therefore, a product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is in the range of 0.30 micrometer to 0.40 micrometer in a device having a VA mode, and in the range of 0.20 micrometer to 0.30 micrometer in a device having an IPS mode. In the above case, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large absolute value of a dielectric anisotropy of the liquid crystal composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large absolute value of the dielectric anisotropy is preferred.
A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in the device. Accordingly, a composition having a large specific resistance, at room temperature and also at a high temperature in an initial stage, is preferred. A composition having a large specific resistance, at room temperature and also at a high temperature even after the device has been used for a long period of time, is preferred.
Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display element. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A composition having a positive dielectric anisotropy is used for an AM device having a TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device to which a polymer sustained alignment (PSA) technology is applied.
For the reason stated above, an AM device desirably has characteristics such as a wide temperature range in which the device can be used, a short response time, a large contrast ratio, a low threshold voltage, a large voltage holding ratio and a long service life. In particular, an AM device desirably has a shorter response time even by one millisecond and an unlikely poor display regarding unevenness, image sticking and so forth.
On the other hand, a technical development of the liquid crystal display element has been achieved not only by improvement of the driving mode or structure of the liquid crystal display element but also by improvement of component parts used for the liquid crystal display element.
The liquid crystal display element ordinarily has an alignment layer for aligning the liquid crystal composition in a liquid crystal layer in a specific direction. The alignment layer is one of important elements involved in a display quality level of the liquid crystal display element, and a role of the alignment layer has become increasingly important every year along with a development of a high quality liquid crystal display element.
The alignment layer is prepared from an aligning agent. The aligning agent mainly used currently includes a solution prepared by dissolving a polyamic acid or soluble polyimide in an organic solvent. The alignment layer is formed by applying such a solution to a substrate and then performing film formation by a means such as heating.
For the alignment layer subjected to film formation, anisotropy is exhibited by applying a rubbing method for rubbing the layer in a specific direction with a fabric of rayon, cotton and so forth, and thus liquid crystals are aligned in a predetermined direction.
However, the method for controlling alignment by rubbing includes the problematic points as described below.
1) Because a rubbing method includes a mechanical technique, a precise adjustment of an initial alignment state of the liquid crystals is difficult, and exhibition of mutually different alignment directions is difficult in a fine region.
2) A scratch and a flaw generated during rubbing cause a decrease in yield.
3) When a drive voltage is applied to a liquid crystal display apparatus and then the drive voltage is turned off after alignment of liquid crystal molecules has been completed, and then a drive voltage is applied again, a final alignment state of the liquid crystal molecules is different from a state during the last drive voltage application. Therefore, the final alignment state of the liquid crystal molecules irregularly changes every time a drive voltage is applied, and thus a long time is required until the liquid crystal molecules reach the final alignment state. Accordingly, a response time of the liquid crystals becomes long.
4) In the case of the VA mode, a viewing angle worsens by using alignment in a single direction.
In order to solve a problem of the rubbing method, a method for improving a response speed and a contrast has been proposed particularly by using a polymer sustained alignment (PSA) technology for a VA display device.
According to the PSA mode, addition of a monomer having a high reactivity is needed to a liquid crystal material in which a high holding ratio is required (FIG. 2 and Patent literatures No. 1 to No. 5). However, addition of a reactive compound to the liquid crystal material may cause an unreacted monomer to remain in the liquid crystal material, and may result in a poor liquid crystal display.
On the other hand, a method for allowing a reactive monomer to be contained in an aligning agent and polymerizing the monomer by applying voltage has been proposed as a method for improving the disadvantage (Non-patent literature No. 1).